The present invention relates to a planar optical waveguide device for the processing of optical signals and its construction.
The utilisation of optical circuits is becoming increasingly important in the transmission of high bandwidth telecommunication signals. Further, fast switching, wavelength selection and wavelength filtering are rapidly becoming indispensable components of all optical Dense Wavelength Division Multiplexed (DWDM) networks. Such devices tend to be exceedingly complex and expensive to construct.
Another very important optical device at the heart of the DWDM systems is a multi wavelength transmitter. Among available candidates for multi wavelength transmitters, are those based on external cavity silica-based laser modules which offer low chirp and greater wavelength stability and control. Unfortunately, such devices often have minimal modulation capabilities. The desirable inclusion of optical signal modulation functions in such modules would enhance their performance and increase the cost effectiveness of fabrication.
Known carrier injection/absorption effects in semiconductors permit switching/modulation to be performed on the required time scale of nanoseconds or less. Existing devices incorporating semi-conductor functionality are typically epitaxially grown, which involves high growth temperatures, or fabricated in monocrystalline semiconductor substrates. Whilst having high modulation/switching performance such devices are relatively expensive to fabricate, suffer high insertion losses because of the utilisation of the semi-conductor waveguide material, suffer cross talk and are limited in the number of wavelength channels.
Recently, a hybrid solution has been proposed where integrated semiconductor optical amplifier (SOA) gates which perform switching are integrated on a platform with silica-based waveguides used for signal routing and fibre interconnects. Although this solution combines advantages of fast switching in semiconductors with the versatility of silica-based waveguides, it is still not ideal from the point of view of mass manufacturability and cost, since the SOA gates still need to be manufactured on separate chips and then assembled in modules, requiring alignment and wire bonding.
In accordance with a first aspect of the present invention; there is provided a device for processing an optical signal, the device comprising:
an optical processing element substantially embedded in a silica-based material; and
a planar optical waveguide structure arranged to guide the optical signal, the waveguide structure comprising a silica-based core;
wherein the optical processing element and the planar optical waveguide are monolithically integrated.
The processing element may comprise a first portion and a control means for altering a refractive index of the first portion so as to modify the optical signal. The control means may comprise electrodes arranged to enable charged carriers to be injected into the first portion of the processing element so as to alter the refractive index of the first portion.
The processing element may be arranged to modify an effective refractive index of at least one optical mode of the optical signal in a first region of the waveguide structure.
The processing element may be incorporated in or closely adjacent to the core of the waveguide structure. In one embodiment, the first portion of the processing element effectively forms part of the cladding in the first region of the waveguide. When the refractive index of the first portion of the processing element is changed, the effective refractive index of at least one optical mode of the optical signal is also changed. In this case, the processing element functions as a phase modulator.
The processing element may comprise a semiconductor-based component. The semiconductor-based component can incorporate silicon. The semiconductor-based component may incorporate hydrogenated amorphous silicon. Alternatively, the semiconductor-based component may incorporate polycrystalline silicon.
Alternatively, the processing element may comprise an electro-optic material for modulating the optical signal. The material may have a high "khgr"(2). In one embodiment, the material comprises barium titanate.
A geometry and spatial relationship of the processing element and the waveguide structure may be chosen such that, in use, optical signal losses associated with optical mode transmission between the processing element and the waveguide structure are reduced.
The optical processing element may alternatively be arranged to convert at least a portion of the optical signal into a corresponding electrical signal. In this case, the processing element may incorporate a semiconductor-based component. The semiconductor-based component may have a bandgap selected to exhibit optical absorption at a wavelength of the optical signal. The semiconductor-based component may incorporate hydrogenated silicon and hydrogenated germanium.
In all of the above-described embodiments, the waveguide structure and the processing element may be monolithically integrated on a substrate comprising electrical circuits for the processing element. The substrate may incorporate a silicon wafer.
In accordance with a second aspect of the present invention, there is provided a method of fabricating any one of the devices described above, wherein hollow cathode chemical vapour deposition is used to form the waveguide structure.
At least a portion of the silica-based waveguide structure may be formed after forming the processing element.
The processing element may comprise a semiconductor component formed by solid phase crystallisation of amorphous silicon. Alternatively, the processing element may comprise hydrogenated amorphous silicon deposited by plasma enhanced chemical vapour deposition (PECVD).
The method may comprise removing a portion of a cladding layer of the waveguide structure in order to monolithically integrate the processing element with the waveguide structure.